0–360 longitude to -180–180 longitude convention
360 Frequently, we come across raster (GeoTIFF/NetCDF) files that seem to be in the Latitude/Longitude projection, but their longitude values span from 0 to 360 degrees, instead of using the GIS-standard arrangement of -180 degrees to 180 degrees for longitude centered on the Prime Meridian, in this article we will use the pyramids package to convert the raster file to use the -180 to 180 longitude convention
Data sources that use 0–360 degree longitude convention
Some examples of raster files that use the 0 to 360-degree longitude convention include:
- NOAA Optimum Interpolation Sea Surface Temperature (OISST) dataset — this dataset provides a daily analysis of sea surface temperature on a 0.25-degree grid for the global ocean.
- ECMWF ERA5 climate reanalysis dataset — this dataset provides a range of climate variables on a 0.25-degree grid for the global atmosphere and uses a 0 to 360 degree longitude convention.
- MODIS Level 3 global vegetation index (GVI) dataset — this dataset provides monthly composites of vegetation indices on a 0.05-degree grid.
- AVHRR Global Area Coverage (GAC) dataset — this dataset provides daily composites of sea surface temperature on a 4-km resolution grid for the global ocean.
It’s worth noting that some software and tools may be able to handle both 0 to 360 and -180 to 180 degree longitude conventions, so it’s important to check the data specifications and properties to ensure that the data is properly handled and analyzed
QGIS/ARC GIS Pro
Both QGIS and ARC GIS Pro are capable of opening rasters with longitude values ranging from 0 to 360 degrees. However, by default, they assumes that the data is in the standard latitude/longitude projection, which uses the -180 to 180 degree longitude range. Therefore, if you are working with data that uses 0 to 360 degree longitude values, you may need to adjust the projection settings in QGIS to correctly display the data.
Pyramids python package
- The Pyramids is a Python package, that is built on top of GDAL, pyramids package uses a very similar API to Geopandas API to manipulate raster and vector data.
- To install pyramids using conda
conda install -c conda-forge pyramids- or using pip
pip install pyramids-gis- for more details, you can check the package in GitHub.
Open the raster
- we can now open the raster file using the read_file method from the Dataset class that represents rasters in the pyramids package.
- After you read the raster, you can check the properties of the raster by printing the object.
Lon attribute
- By checking the max value in the lon attribute of the dataset object, you can check whether the raster has the 0–360 longitude convention.
Plot the dataset
- You can also plot the raster using the plot method which plots an RGB, if the raster has more than one band, and you can specify a specific band to plot.
convert_longitude
- Now use the convert_longitude to convert the longitude values to range between -180 and 180.
- Now the longitude ranges from -180 to 180 (cell size is 0.25 degree)
Save the new raster
- Now to save the converted dataset to disk, the data format of the raster will be inferred from the extension at the end of the path given to the to_file method
Check the raster with QGIS
- Now if you open the new raster in QGIS and use any basemap to use it as a reference for the correct location
So, to sum up, in this article, we have converted the 0–360 longitude projection in the NOAA raster to use the standard -180–180 longitude convention using the Pyramids python package and verified the new raster with GQIS.
You can find all the code used in this article in a Jupyter notebook here (https://github.com/Serapieum-of-alex/pyramids/blob/main/examples/notebooks/convert-longitude.ipynb)
